Tracking the position of intrabody objects, such as sensors, tubes, catheters, dispensing devices, and implants, is required for many medical procedures. Well-established, highly accurate systems for determining the position of an intrabody object have been developed based on magnetic field sensing. These systems utilize sensors affixed to the intrabody object to measure the relative strengths of externally-generated magnetic fields and to derive from these measurements the position of the object. Methods for magnetic-based position sensing are disclosed, for example, in U.S. Pat. Nos. 5,391,199, 5,443,489, and 6,788,967 to Ben-Haim, in U.S. Pat. No. 6,690,963 to Ben-Haim, et al., in U.S. Pat. No. 5,558,091 to Acker et al., in U.S. Pat. No. 6,172,499 to Ashe, and in U.S. Pat. No. 6,177,792 to Govari, all of whose disclosures are incorporated herein by reference.
Position sensing systems have also been developed which utilize impedance-based measurements. In such systems, impedance is measured between electrodes affixed to the intrabody object and electrodes placed on the body surface. The systems then derive the position of the intrabody object from the impedance measurements. Methods for impedance-based position sensing are disclosed, for example, in U.S. Pat. No. 5,983,126 to Wittkampf, in U.S. Pat. No. 6,456,864 to Swanson, and in U.S. Pat. No. 5,944,022 to Nardella, all of whose disclosures are incorporated herein by reference.
Impedance-based position sensing is generally less expensive to implement than magnetic field position sensing. Many standard catheters, such as those used for electrophysiological mapping and ablation, already incorporate electrodes that can be utilized for impedance measurements. However, due in part to the non-linear impedance of the body, impedance-based position sensing is not as accurate as magnetic-based methods.
U.S. Pat. No. 6,574,498 to Gilboa, whose disclosure is incorporated herein by reference, describes a method of intrabody navigation that relies on an electromagnetic technique to determine the position and orientation of the patient relative to an imaging device, while using another technique, such as the ultrasonic or electrical impedance sensing, to determine the position and orientation of a probe relative to the patient's body. The method includes determining a position and an orientation of the probe relative to a primary coordinate system and to a secondary coordinate system, and determining a transformation from the secondary coordinate system to the primary coordinate system.
U.S. Pat. No. 5,899,860 to Pfeiffer, et al., whose disclosure is incorporated herein by reference, describes a method for determining the position of a catheter inside the body of a patient. A correction function is determined by having a catheter perform a known movement inside a body cavity at the same time as the position of the catheter is determined from position signals sent between the catheter and a remote position location. Subsequent catheter positions, derived from received location signals, are corrected according to the correction function.
Magnetic-based position sensing systems currently available include proprietary products such as the CARTO™ EP Navigation and Ablation System and the LASSO™ Circular Mapping Catheter from Biosense-Webster (Diamond Bar, Calif.).